A process of this kind and a corresponding device are known from DE-OS 34 40 437.
Crack detecting processes are required in particular in the mass production of components that are particularly important from the point of view of safety, as surface cracks form a considerable danger. This is particularly the case for motor vehicle components which are manufactured in large quantities but have to be examined individually for the presence of cracks. The visual evaluation of defect indications by an inspector is a considerable strain since the probability of defects occurring is very small and this affects the inspector's ability to concentrate. As a result, the probability of defect detection suffers.
In the known process the inspection is carried out automatically by digitising the picture information of the camera picture of the grey shades and storing the digital values obtained in this way as a matrix of picture elements (pixels) in a semiconductor store. By means of a computer, in a first processing step, a binary picture is produced from this and in a second processing step all indications of linear and punctiform structures are eliminated whose extent does not exceed a preselected number of picture elements. In a third processing step the remaining indications of surface structures are enlarged in each image direction to a size exceeding their original extent by at least one picture element; the binary picture obtained in this way is extracted from the output binary picture as a mask covering all surface structures. Finally, the remaining binary picture is scanned in rows or columns for evaluation. The picture elements comprising indications that are detected by this means are counted, and a sorting report is triggered if, when a series of picture elements directly adjoining one another is scanned, a preselected minimum number of indications have been counted.
Since in the known process a binary picture is produced in the first step there is no simple way of improving the signal-to-noise ratio. In addition, the binary picture produced according to the known process is not independent of the absolute brightness of the original picture, nor can changes in the installation be indicated, for example a decrease in the intensity of illumination, a change in the composition of the crack representing agent or the effect of dust. The use of a black and white television camera also greatly restricts the possibility of analog signal preprocessing.